Heat exchangers such as radiators, condensers, and evaporators to be mounted to automobiles are often prepared by forming, assembling, and brazing aluminum alloy sheet materials, because such aluminum alloy materials are lightweight and have satisfactory thermal conductivity. Such aluminum alloy sheets, when formed into tubes typically of radiators, for example, are exposed to the atmosphere (air) on the outer surface and exposed to a coolant such as cooling water on the inner surface. When the aluminum alloy sheets are exposed to these corrosive surroundings, corrosion (pitting corrosion) may locally proceed and may lead to the formation of through holes. For corrosion protection of the outer surface of the tube, so-called “cathodic protection” (electrolytic protection) is generally adopted and is known to be effective. In the cathodic protection, a fin material is brought into contact with the outer surface of the tube, which fin material is composed of an Al—Zn alloy or another substance being less noble in potential than the aluminum alloy constituting the tube. The cathodic protection is often adopted also for the corrosion protection of the inner surface of the tube. Specifically, such a tube is generally formed from a clad material, which clad material includes a base material (core material) composed of an aluminum alloy, and a sacrificial anode material cladded on the inner surface of the core material and composed of an Al—Zn alloy which is less noble in potential than the aluminum alloy of the core material. The clad material is often a clad material (blazing sheet) which further includes a filler material cladded on the outer surface of the core material and has three or more layers. The filler material is composed of an Al—Si alloy and contributes to the blazing typically with a fin material. Independently, such a clad material should have a thin wall thickness (gauge) of about 0.3 mm or less, from the viewpoint of reducing the weight of an apparatus to which the clad material is adopted.
Such aluminum alloy clad materials have been improved in the following manner so as to provide aluminum alloy materials which have satisfactory strength and corrosion resistance necessary for heat exchangers even when formed into thin members and are thereby usable in heat exchangers. For example, Japanese Unexamined Patent Application Publication (JP-A) No. H11(1999) -61306 discloses a three-layered clad material including an aluminum alloy core material, a filler material on one side of the core material, and a sacrificial anode material on the other side of the core material, in which the sacrificial anode material is composed of an aluminum alloy containing 1.0 to 6.0 percent by mass of zinc (Zn) and further containing manganese (Mn). In the sacrificial anode material, the grain sizes (particle diameters) and distribution of Al—Mn intermetallic compounds are controlled. This reduces the corrosion current of the sacrificial anode material to thereby improve the corrosion resistance of the resulting three-layered clad material. In this technique, the grain sizes and distribution of Al—Mn intermetallic compounds are controlled by adjusting the temperature of a homogenization treatment of ingots of aluminum alloy as the raw material for the sacrificial anode material and the temperature of clad rolling. Japanese Unexamined Patent Application Publication (JP-A) No. 2006-131923 discloses a four-layered material (brazing sheet) which includes a core material composed of an aluminum alloy containing predetermined amounts of Mn and Mg; an intermediate layer cladded on one surface of the core material and composed of an aluminum alloy containing Mn; a filler material cladded on the intermediate layer; and a sacrificial anode material cladded on the other surface of the core material and composed of an aluminum alloy containing 0.5 to 10 percent by mass of Zn and further containing Mn. This technique is intended to improve the strength by adding Mn not only to the core material but also to the sacrificial anode material. The technique is also intended to prevent the generation of local pitting corrosion, by dispersing Al—Mn intermetallic compounds in the sacrificial anode material and thereby allowing the dispersed Al—Mn intermetallic compounds to cause pitting corrosion not locally but uniformly (generally). Japanese Unexamined Patent Application Publication (JP-A) No. 2006-176852 discloses a three-layered clad material including a sacrificial anode material containing 0.2 to 8.0 percent by mass of Zn and further containing Sc and at least one of Mn, Fe, Si, Cu, Mg, and Zr so as to have improved strength and erosion resistance. The addition of these elements allows the aluminum alloy clad material to have further higher resistance both to local corrosion and to general corrosion.